diff --git a/src/client.ts b/src/client.ts
index 608b318..e866d01 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -17,6 +17,7 @@ import * as Errors from './core/error';
 import * as Pagination from './core/pagination';
 import { AbstractPage, type SkipLimitParams, SkipLimitResponse } from './core/pagination';
 import * as Uploads from './core/uploads';
+import { maybeMultipartFormRequestOptions } from './internal/uploads';
 import * as API from './resources/index';
 import { APIPromise } from './core/api-promise';
 import {
@@ -393,6 +394,12 @@ export class PostGrid {
 
     await this.prepareOptions(options);
 
+    // if body contains file uploads, convert to multipart/form-data (fastest way and making compiler happy)
+    const withMultipart = await maybeMultipartFormRequestOptions(options, this);
+    if (withMultipart.body !== options.body) {
+      options.body = withMultipart.body;
+    }
+
     const { req, url, timeout } = await this.buildRequest(options, {
       retryCount: maxRetries - retriesRemaining,
     });
diff --git a/src/resources/print-mail/letters.ts b/src/resources/print-mail/letters.ts
index 9fc64c0..ef24147 100644
--- a/src/resources/print-mail/letters.ts
+++ b/src/resources/print-mail/letters.ts
@@ -1,6 +1,7 @@
 // File generated from our OpenAPI spec by Stainless. See CONTRIBUTING.md for details.
 
 import { APIResource } from '../../core/resource';
+import type { Uploadable } from '../../core/uploads';
 import * as ContactsAPI from './contacts';
 import * as PrintMailAPI from './print-mail';
 import { APIPromise } from '../../core/api-promise';
@@ -108,7 +109,7 @@ export interface AttachedPdf {
   /**
    * The file (multipart form upload) or URL pointing to a PDF for the attached PDF.
    */
-  file: string;
+  file: string | Uploadable;
 
   /**
    * Enum representing the placement of the attached PDF.
@@ -405,7 +406,7 @@ export namespace PlasticCard {
      * A URL pointing to a PDF file for the double-sided plastic card or the file
      * itself.
      */
-    pdf?: string;
+    pdf?: string | Uploadable;
   }
 
   /**
@@ -422,7 +423,7 @@ export namespace PlasticCard {
      * A URL pointing to a PDF file for the single-sided plastic card or the PDF file
      * itself.
      */
-    pdf?: string;
+    pdf?: string | Uploadable;
 
     /**
      * The template ID for the single-sided plastic card.
@@ -596,7 +597,7 @@ export declare namespace LetterCreateParams {
     /**
      * A URL pointing to a PDF file for the letter or the PDF file itself.
      */
-    pdf: string;
+    pdf: string | Uploadable;
 
     /**
      * The recipient of this order. You can either supply the contact information
diff --git a/src/resources/print-mail/order-profiles/cheques.ts b/src/resources/print-mail/order-profiles/cheques.ts
index 7bd51e6..ad6e2c6 100644
--- a/src/resources/print-mail/order-profiles/cheques.ts
+++ b/src/resources/print-mail/order-profiles/cheques.ts
@@ -1,6 +1,7 @@
 // File generated from our OpenAPI spec by Stainless. See CONTRIBUTING.md for details.
 
 import { APIResource } from '../../../core/resource';
+import type { Uploadable } from '../../../core/uploads';
 import * as ChequesAPI from '../cheques';
 import { APIPromise } from '../../../core/api-promise';
 import { PagePromise, SkipLimit, type SkipLimitParams } from '../../../core/pagination';
@@ -387,7 +388,7 @@ export interface ChequeCreateParams {
    * Body param: PDF file for an optional attached letter. Cannot be used with
    * `letterHTML` or `letterTemplate`. Input only.
    */
-  letterPDF?: string;
+  letterPDF?: string | Uploadable;
 
   /**
    * Body param: ID of a template for an optional attached letter. Cannot be used
@@ -494,7 +495,7 @@ export interface ChequeUpdateParams {
    * Body param: PDF file for an optional attached letter. Cannot be used with
    * `letterHTML` or `letterTemplate`. Input only.
    */
-  letterPDF?: string;
+  letterPDF?: string | Uploadable;
 
   /**
    * Body param: ID of a template for an optional attached letter. Cannot be used
diff --git a/src/resources/print-mail/order-profiles/letters.ts b/src/resources/print-mail/order-profiles/letters.ts
index bf601bd..1b35ea5 100644
--- a/src/resources/print-mail/order-profiles/letters.ts
+++ b/src/resources/print-mail/order-profiles/letters.ts
@@ -1,6 +1,7 @@
 // File generated from our OpenAPI spec by Stainless. See CONTRIBUTING.md for details.
 
 import { APIResource } from '../../../core/resource';
+import type { Uploadable } from '../../../core/uploads';
 import * as LettersAPI from '../letters';
 import { APIPromise } from '../../../core/api-promise';
 import { PagePromise, SkipLimit, type SkipLimitParams } from '../../../core/pagination';
@@ -337,7 +338,7 @@ export interface LetterCreateParams {
    * Body param: A PDF file containing the letter content. Cannot be used with
    * `template`.
    */
-  pdf?: string;
+  pdf?: string | Uploadable;
 
   /**
    * Body param: Specifies which page number should be perforated (if any).
@@ -446,7 +447,7 @@ export interface LetterUpdateParams {
    * Body param: A PDF file containing the letter content. Cannot be used with
    * `template`.
    */
-  pdf?: string;
+  pdf?: string | Uploadable;
 
   /**
    * Body param: Specifies which page number should be perforated (if any).
diff --git a/src/resources/print-mail/order-profiles/postcards.ts b/src/resources/print-mail/order-profiles/postcards.ts
index 02a2ee5..76149e5 100644
--- a/src/resources/print-mail/order-profiles/postcards.ts
+++ b/src/resources/print-mail/order-profiles/postcards.ts
@@ -1,6 +1,7 @@
 // File generated from our OpenAPI spec by Stainless. See CONTRIBUTING.md for details.
 
 import { APIResource } from '../../../core/resource';
+import type { Uploadable } from '../../../core/uploads';
 import { APIPromise } from '../../../core/api-promise';
 import { PagePromise, SkipLimit, type SkipLimitParams } from '../../../core/pagination';
 import { RequestOptions } from '../../../internal/request-options';
@@ -296,7 +297,7 @@ export interface PostcardCreateParams {
    * Body param: A 2-page PDF file containing the postcard content (front and back).
    * Cannot be used with `frontTemplate`/`backTemplate`.
    */
-  pdf?: string;
+  pdf?: string | Uploadable;
 }
 
 export interface PostcardRetrieveParams {
@@ -376,7 +377,7 @@ export interface PostcardUpdateParams {
    * Body param: A 2-page PDF file containing the postcard content (front and back).
    * Cannot be used with `frontTemplate`/`backTemplate`.
    */
-  pdf?: string;
+  pdf?: string | Uploadable;
 }
 
 export interface PostcardListParams extends SkipLimitParams {
diff --git a/src/resources/print-mail/order-profiles/self-mailers.ts b/src/resources/print-mail/order-profiles/self-mailers.ts
index f27b2f4..1834325 100644
--- a/src/resources/print-mail/order-profiles/self-mailers.ts
+++ b/src/resources/print-mail/order-profiles/self-mailers.ts
@@ -1,6 +1,7 @@
 // File generated from our OpenAPI spec by Stainless. See CONTRIBUTING.md for details.
 
 import { APIResource } from '../../../core/resource';
+import type { Uploadable } from '../../../core/uploads';
 import { APIPromise } from '../../../core/api-promise';
 import { PagePromise, SkipLimit, type SkipLimitParams } from '../../../core/pagination';
 import { RequestOptions } from '../../../internal/request-options';
@@ -303,7 +304,7 @@ export interface SelfMailerCreateParams {
    * Body param: A 2-page PDF file containing the self-mailer content (inside and
    * outside). Cannot be used with `insideTemplate`/`outsideTemplate`.
    */
-  pdf?: string;
+  pdf?: string | Uploadable;
 }
 
 export interface SelfMailerRetrieveParams {
@@ -387,7 +388,7 @@ export interface SelfMailerUpdateParams {
    * Body param: A 2-page PDF file containing the self-mailer content (inside and
    * outside). Cannot be used with `insideTemplate`/`outsideTemplate`.
    */
-  pdf?: string;
+  pdf?: string | Uploadable;
 }
 
 export interface SelfMailerListParams extends SkipLimitParams {
diff --git a/src/resources/print-mail/postcards.ts b/src/resources/print-mail/postcards.ts
index 64ea80a..0720cea 100644
--- a/src/resources/print-mail/postcards.ts
+++ b/src/resources/print-mail/postcards.ts
@@ -1,6 +1,7 @@
 // File generated from our OpenAPI spec by Stainless. See CONTRIBUTING.md for details.
 
 import { APIResource } from '../../core/resource';
+import type { Uploadable } from '../../core/uploads';
 import * as ContactsAPI from './contacts';
 import * as PrintMailAPI from './print-mail';
 import * as OrderProfilesPostcardsAPI from './order-profiles/postcards';
@@ -404,7 +405,7 @@ export declare namespace PostcardCreateParams {
      * A URL pointing to a 2 page PDF file. The first page is the front of the postcard
      * and the second page is the back (where the address will be stamped on).
      */
-    pdf: string;
+    pdf: string | Uploadable;
 
     /**
      * Enum representing the supported postcard sizes.
@@ -489,7 +490,7 @@ export declare namespace PostcardCreateParams {
      * A 2 page PDF file. The first page is the front of the postcard and the second
      * page is the back (where the address will be stamped on).
      */
-    pdf: string;
+    pdf: string | Uploadable;
 
     /**
      * Enum representing the supported postcard sizes.
diff --git a/src/resources/print-mail/self-mailers.ts b/src/resources/print-mail/self-mailers.ts
index faccc37..b45754e 100644
--- a/src/resources/print-mail/self-mailers.ts
+++ b/src/resources/print-mail/self-mailers.ts
@@ -1,6 +1,7 @@
 // File generated from our OpenAPI spec by Stainless. See CONTRIBUTING.md for details.
 
 import { APIResource } from '../../core/resource';
+import type { Uploadable } from '../../core/uploads';
 import * as ContactsAPI from './contacts';
 import * as PrintMailAPI from './print-mail';
 import * as OrderProfilesSelfMailersAPI from './order-profiles/self-mailers';
@@ -412,7 +413,7 @@ export declare namespace SelfMailerCreateParams {
      * self-mailer and the second page is the outside (where the address will be
      * stamped on).
      */
-    pdf: string;
+    pdf: string | Uploadable;
 
     /**
      * Enum representing the supported self-mailer sizes.
@@ -496,7 +497,7 @@ export declare namespace SelfMailerCreateParams {
      * A 2 page PDF file. The first page is the inside of the self-mailer and the
      * second page is the outside (where the address will be stamped on).
      */
-    pdf: string;
+    pdf: string | Uploadable;
 
     /**
      * Enum representing the supported self-mailer sizes.
diff --git a/tests/uploads.test.ts b/tests/uploads.test.ts
index 6a964a1..bf63d18 100644
--- a/tests/uploads.test.ts
+++ b/tests/uploads.test.ts
@@ -1,6 +1,7 @@
 import fs from 'fs';
 import type { ResponseLike } from 'postgrid-node/internal/to-file';
 import { toFile } from 'postgrid-node/core/uploads';
+import { maybeMultipartFormRequestOptions } from 'postgrid-node/internal/uploads';
 import { File } from 'node:buffer';
 
 class MyClass {
@@ -105,3 +106,56 @@ describe('missing File error message', () => {
     );
   });
 });
+
+describe('maybeMultipartFormRequestOptions', () => {
+  it('returns opts unchanged when body has no uploadable value', async () => {
+    const body = {
+      from: 'contact_1',
+      to: 'contact_2',
+      pdf: 'https://example.com/letter.pdf',
+    };
+    const opts = { body };
+    const result = await maybeMultipartFormRequestOptions(opts, fetch);
+    expect(result).toBe(opts);
+    expect(result.body).toBe(body);
+  });
+
+  it('returns new opts with FormData body when body contains a File', async () => {
+    const pdfFile = new File(['fake content'], 'letter.pdf', { type: 'application/pdf' });
+    const body = {
+      from: 'contact_1',
+      to: 'contact_2',
+      pdf: pdfFile,
+    };
+    const opts = { body };
+    const result = await maybeMultipartFormRequestOptions(opts, fetch);
+    expect(result).not.toBe(opts);
+    expect(result.body).toBeInstanceOf(FormData);
+    const form = result.body as FormData;
+    expect(form.get('from')).toBe('contact_1');
+    expect(form.get('to')).toBe('contact_2');
+    const pdfPart = form.get('pdf');
+    expect(pdfPart).toBeInstanceOf(Blob);
+    expect((pdfPart as File).name).toBe('letter.pdf');
+  });
+
+  it('works with toFile(Buffer) for PDF upload path', async () => {
+    const buffer = Buffer.from('fake content');
+    const pdfFile = await toFile(buffer, 'letter.pdf');
+    const body = {
+      from: 'contact_1',
+      to: 'contact_2',
+      pdf: pdfFile,
+    };
+    const opts = { body };
+    const result = await maybeMultipartFormRequestOptions(opts, fetch);
+    expect(result).not.toBe(opts);
+    expect(result.body).toBeInstanceOf(FormData);
+    const form = result.body as FormData;
+    expect(form.get('from')).toBe('contact_1');
+    expect(form.get('to')).toBe('contact_2');
+    const pdfPart = form.get('pdf');
+    expect(pdfPart).toBeInstanceOf(Blob);
+    expect((pdfPart as File).name).toBe('letter.pdf');
+  });
+});